build: status of the special eoywn mode

[Manlio Perillo]

By eowyn mode I mean the code that builds all the exercises using full parallelism, without using ZiglingStep.
I added it in order to improve the performance of the Eoywn workflow, but I disabled it because the unit tests also use zig build -Dhealed.

The code can be enabled again by exporting, e.g., the HEALED_BY_EOWYN environment variable in the patches/eowyn.sh script.

A better solution is to simply remove the code, simplifying build.zig.

@ratfactor, @chrboesch What do you think?

[Chris Boesch]

@perillo I think it is not worth for a bit more speed to support different build modes. So in my opinion you can remove the code.

[Dave Gauer]

I agree. Deleted code is the best kind.